How to determine order of reaction?

How do we deduce the order of reaction?

The syllabus specify that students should know how to deduce the order of reaction with respect to a particular reactant using the initial rates method. Initial rate simply means the rate of reaction at the start of the reaction. For a typical chemical reaction, the initial rate of reaction is the fastest because the rate of reaction slows down as the reactants are being used up. Hence we would use the initial rate of reaction to represent the "true" rate of reaction.

Let's consider the reaction A + B --> C
The initial rates of reaction were determined and the results were presented in the table below


First we would determine the order of reaction with respect to reactant A. To do that we would look at Run(a) and Run(b) because the concentration of the other reactant B is the same. Only the concentration of A is changed. Hence whatever change in rate of reaction observed is due to A. Comparing Run(a) and run(b), we see that [A] is doubled however the reaction rate remains unchanged. Hence the order of reaction w.r.t A is zero order.

Secondly, we would dtermine the order of reaction w.r.t reactant B. Using the same reasoning we compare Run(b) and (c) where the [A] remain constant and [B] changes. We realise that the reaction rate doubles when the [B] doubles. Hence the order of reaction with respect to reactant B is first order.